Historically, spectroscopy has referred to the study of wavelengths of visible light. Today, the term is used to refer to almost any measurement of a quantity as a function of either wavelength or frequency. Spectroscopy is often used in the field of astronomy and other remote sensing applications.
Raman spectroscopy is a technique used to study vibrational, rotational, and other low-frequency modes in a system. In practice, Raman spectroscopy relies on inelastic scattering of monochromatic light, e.g., via a laser. Upon interaction with molecules, the energy of incident laser photons is shifted up or down depending on the characteristic frequencies of the molecules excited. This shift in energy can be analyzed to yield information about the molecules in the system.
Spontaneous Raman scattering (SRS) is a linear scattering phenomena, which is typically very weak. As a result, one difficulty of Raman spectroscopy is separating the weak inelastically scattered light from the intense Rayleigh scattered laser light. Traditionally, holographic gratings and multiple dispersion stages were used in Raman spectrometers to achieve a high degree of laser rejection. Additionally, photomultipliers were often used as detectors for dispersive Raman setups. However, these photomultipliers were inefficient and resulted in poor spectral resolution. Today, instrumentation in Raman spectroscopy utilizes notch or edge filters for laser rejection and spectrographs.
One of the critical aspects of time-resolved SRS spectroscopy in combustion diagnostics is implementing a temporal gating scheme to reject optical background, thus increasing signal-to-noise ratio (SNR). Traditionally, experimenters have had two options: (1) electronic gating using an image intensifier; or (2) a mechanical shutter.
Many have chosen the image intensified CCD (charge-coupled device) (or ICCD) for electronic gating due to its superior nanosecond gating speed. However, there are several drawbacks with an ICCD, such as compromised image quality, lower dynamic range, and the possibility of permanent damage due to inadvertent bright sources of light.
A high-speed mechanical shutter by way of rotary optical choppers can provide a <10 μs gate at <30 Hz. With such a shutter system, one can take full advantage of the high dynamic range and quantum efficiency offered by conventional back-illuminated CCDs. On the other hand, 10 μs gating may not be acceptable for applications with higher levels of background. The use of a shutter often results in transmission losses of up to 50% due to internal relay optics and inherent timing jitter.
Regardless of the type of gating employed, laser-generated interference, such as laser-induced fluorescence (LIF) of C2 carbons or polycyclic aromatic hydrocarbons (PAH), may still be problematic since these emissions occur simultaneously with Raman scattering. This problem is universal to all laser spectroscopy techniques, not just Raman diagnostics in combustion.